The present invention relates to a temperature control system for lamp annealers.
Lamp annealers are adapted to heat an article as placed, for example, in a processing tube made of quartz by irradiating the article with the light of halogen lamps, and are used, for example, for annealing semiconductor wafers.
The lamp annealer for annealing wafers are so controlled that the temperature of the wafer is in match with a predetermined target temperature control pattern. FIG. 5 shows an example of target temperature control pattern for annealing wafers. In this case, the temperature of the wafer reaches a constant temperature level of about 1000.degree. C. which is the desired or target temperature about 8 seconds after the start of heating, and the wafer is gradually cooled about 30 seconds after the start of heating. In annealing wafers in this way, the temperature of the annealer must be controlled with high accuracy with a tolerance of not greater than .+-.2.degree. C. especially at the constant temperature level.
The methods of temperature control according to such a target temperature control pattern include open-loop control and closed-loop control. In the former method, the lamp output is controlled according to a lamp output control pattern which is predetermined as by experiments. In the latter method, the lamp output is controlled while measuring the temperature of wafer with a temperature sensor so that the measured temperature will match a target temperature control pattern. These methods, however, have the following problems.
While the temperature of the wafer is dependent on the amount of energy given by the lamps, differences in the initial temperature of the processing tube and treating gas and the initial temperature and absorption factor of the wafer result in different modes of rise of temperature even if the same amount of energy is given, so that it is impossible for the open-loop method to control the wafer temperature in match with the targent temperature control pattern with good reproducibility.
In the case of closed-loop control, on the other hand, it is required to measure the wafer temperature over a wide range without a time lag, whereas conventional temperature sensors, such as thermocouples radiation thermometers, encounter difficulty in meeting this requirement. The method is therefore unable to control the wafer temperature in conformity with the target temperature control pattern. Stated more specifically, the temperature of the wafer to be annealed rises rapidly, whereas it requires some time to thermally equilibrate the thermocouple with the wafer such that the temperature rises before the thermocouple comes to thermal equilibrium with the wafer. Such a great time lag of the thermocouple behind the temperature change of the wafer presents difficulty in matching the actual wafer temperature with the target temperature control pattern. The radiation thermometer is small in time lag but is limited in measuring range, and is unable to measure temperatures of up to about 300.degree. C. Accordingly, it is also difficult to use the radiation thermometer for closed-loop control.
To overcome these problems, a temperature control system for lamp annealers has been proposed which employs a radiation thermometer and which is adapted to control the lamp output by the open-loop method according to a predetermined lamp output control pattern until the temperature reaches a range wherein the radiation thermometer is usable for measurement and to thereafter control the lamp output by the closed-loop method so that the measured temperature will match a target temperature control pattern (see Unexamined Japanese Patent Publication SHO 62-98722).
When the proposed system operates for open-loop control before the temperature reaches the range permitting measurement with the radiation thermometer, the mode of rise of temperature differs, for example, with the condition of the wafer as previously stated. For this reasion, the gradient of temperature measured when the open-loop control is changed over to the closed-loop control upon the temperature reaching the range measurable with the radiation thermometer also differs depending on the condition of the wafer. If the gradient of measured temperature obtained at this time is greatly different from the gradient of the target temperature control pattern, the temperature actually measured involves a steady-state deviation from the target temperature. The sytem then fails to effect accurate control.